The invention discloses a UPS (Uninterrupted Power System), and the system comprises an air circuit breaker Q1, an air circuit breaker Q2, a maintenance bypass air switch Q3BP, a bypass static switch, a rectification module REC, an inversion module INV, an output isolation transformer TR2, a main circuit static switch, an air circuit breaker Q5, a contact-type normally-opened contact KM, and a storage battery. A main input circuit AC power supply is inputted from the air circuit breaker Q1, and the AC power supply is converted into a DC power supply through the rectification module REC, thereby completing the AC/DC conversion, so as to charge the storage battery in parallel connection with a main input circuit and to provide a reliable input direct current for the inversion module INV. The inversion module INV carries out the DC/AC conversion, and converts the DC power supply provided by the rectification module REC and the storage battery into the AC power supply. The AC power supply is outputted through the output isolation transformer TR2. A bypass input power supply is inputted from the air circuit breaker Q2, and is controlled by the bypass static switch and then is outputted. The UPS is stable in voltage, is quick and efficient, is simple in operation, and is convenient to maintain.